A pilot study in a surgically post-menopausal female was carried out using the naturally occuring calcium radio-isotope, 41Ca (half life 10/5 yr). Accelerator mass spectrometric measurements, made in collaboration with Stewart Freeman of Lawrence Livermore National Laboratory, of 41Ca were coupled with results from a typical stable isotope dilution study carried out simultaneously. Values of calcium turnover were calculated from both the stable and 41Ca tracer studies were calculated 1. to be 220 and 234 mg/d respectively. These results are a successful demonstration of the ability to obtain comparable results from steady-state and dynamic appraoches in a complex protocol. This validation will increase confidience in the use of the steady state approach for studies of calcium skeletal dynamics at long times after the administration of the 41Ca tracer, i.e., the subject's lifetime and has led directly to planning of more medically significant studies. 2. The effects of estrogen replacement on calcium metabolism in 6 hypogonadal females, 5 with Turner's Syndrome and one with hypogonadotropic hypogonadism in paired pre- and post-treatment studies of Ca metabolism. We found a significant increase in whole body Ca retention, Vbal changing from 3.6 plus minus 1.3 to 10.2 plus minus 9.2 mg/d, p=0.04, and a significant decrease in whole body turnover from 74.6 plus minus 11.7 to 65.2 plus minus 9.7 mg/d, significant, p=0.04. Based on the results of this work, we speculate that estrogen, even at the virtually unmeasurable levels at which it is secreted from the immature ovary of normal pre-pubescent females, is extremely important for the development of a normal skeleton. The absence of these low levels of hormone during early childhood is closely associated with retardation in normal skeletal development. 3. We devised a method for estimating the fraction of urinary calcium which arises from recent dietary sources. In a study of 38 healthy female children, we have concluded that the contribution of calcium from recent dietary sources is trivial and about averages 8% of absorbed Ca.